Neoplastic transformation produces many changes in cell physiology. To study these changes, we have developed and utilized a new ultrastructural immunocytochemical technique (EGS procedure) to localize over twenty different intracellular protein antigens in cultured cells. The most recent of these studies localized the G glycoprotein of Vesicular Stomatitis Virus, the bristle coat protein clathrin, fibronectin and the calcium-regulatory protein calmodulin. We have also recently combined these localization methods with single cell microinjection techniques to study the processes of receptor-mediated endocytosis and exocytosis, showing that clathrin-coated pits mediate the entry of receptor-ligand complexes but do not form isolated coated vesicles, and that the exocytosis pathway from the Golgi does not involve clathrin coated structures. Also, using cytochemical markers, we have shown that coated pits are stable elements on the plasma membrane, and that isolated coated vesicles do not exist in living cultured fibroblasts.